1. Field of the Invention
The present invention relates to a coating method and, more particularly, to a coating method in a coating line on which substrates to be coated with a water base paint and substrates to be coated with an oil paint are conveyed simultaneously in a mixed state.
2. Description of Related Art
Generally speaking, outer panels or other outer parts of a vehicle, as an automobile, place great importance on their aesthetic appearance so that they are finished by coating them with a thermosetting paint of a type capable of being diluted with an organic solvent, which can provide a final coat having coat properties superior in a smoothness, a gloss, a weather resistance and so on. They are usually finished in many cases by coating such a thermosetting, transparent, clear paint on an air-dried coat obtained by coating them with a thermosetting enamel paint of a type capable of being diluted with an organic solvent, which contains a color pigment and/or a metallic pigment, and by setting two coats simultaneously.
Recently, however, demands to further improve an appearance on a coat surface of outer panels or other outer parts of the vehicle have been made, together with demands to provide a working environment in which no volatile organic solvents are free. In order to meet with these demands, Japanese Patent Publication (Kokai) No. 193,676 discloses a procedure that such a clear paint is coated on an undercoat obtained by coating a water base paint which is conveniently compatible with the clear coat and contains a hazardous volatile ingredient.
As water base paints have been frequently employed for the purpose as have been described hereinabove, a demand has been made to convey coating substrates to be coated with a water base paint simultaneously with coating substrates to be coated with an oil paint in a mixed state on a coating line. In order to deal with such a demand, however, a unique plan should be elaborated so as to compete with a difference in kinds of solvents contained in the paints to be used for coating substrates with the water base paint and with the oil paint. More specifically, the substrates coated with the water base paint, on the one hand, are required to be heated in a pre-heating step to allow the water in the water base paint to evaporate to an appropriate level of dryness. If the substrates coated with the oil paint, on the other hand, would be conveyed to such a pre-heating step immediately after they have been coated with the oil paint, like the substrates coated with the water base paint, volatile solvents contained in conventional oil base paints are caused to be heated to an excessively high temperature and evaporated, thereby forming a rough surface on the coat and, in a worse case, causing a pinhole on the coat surface. This gives rise to the inapplicability of a conventional coating line to a mixed conveyance of the substrates coated with the water base paint together with those coated with the oil paint.
It is to be noted as a matter of course that such problems could never occur if the substrates coated with the water base paint and those coated with the oil paint would be conveyed in a separate coating line. However, a coating line requires a significantly broad space so that a provision of two coating lines presents practically inapplicable problems, each being provided exclusively for the coating with the water base paint and for the coating with the oil paint.